Method of making thin film thermistor

ABSTRACT

A method of manufacturing a thin film thermistor in which the reagent grade materials are dissolved in water and allowed to intimately mix therein after which the mixture is spray-dried under pressure to obtain a fine powder which is the true thermistor composition desired. After oxidation at high temperatures for a long period of time to remove sulphites etc., the resulting oxides are mixed in a bone milling process with a plastic binder which is then poured on to a glass plate and spread to a predetermined thickness and allowed to dry, after which the material is placed on a ceramic plate and fired to bond the plastic and ceramic and burn out portions of the plastic whereupon electrodes are attached to the final thermistor.

United States Patent 91 Sommer Aug. 27, 1974 [54] METHOD OF MAKING mm FILM Primary ExaminerCharles w. Lanham THERMISTOR Assistant Examiner-Victor A. DiPalma [75] Inventor: Alfred Sommer, Teaneck, NJ. fg Flrm Kenyon & Kenyon Remy [73] Assignee: Ceramic Magnetics Inc., Fairfield,

NJ [57] ABSTRACT [22] Filed: Aug. 2, 1973 Appl. No.: 384,949

US. Cl. 29/612, 338/22 R [51] Int. Cl H0lc 7/04 [58] Field of Search 29/612, 621, 182.2, 182.5; 338/22 R, 25; 75/211; 148/126 [56] References Cited UNITED STATES PATENTS 2,633,521 3/1953 Becker et a1 338/22 R 3,364,565 l/l968 Sapoff et a1 29/612 3,444,501 5/1969 Delaney et al 29/612 X 3,555,671 l/l97l Aizicovici 29/612 3,775,843 12/1973 Wendt, .lr. 29/612 A method of manufacturing a thin film thermistor in which the reagent grade materials are dissolved in water and allowed to intimately mix therein 'after which the mixture is spray-dried under pressure to obtain a fine powder which is the true thermistor composition desired. After oxidation at high temperatures for a long period of time to remove sulphites etc., the resulting oxides are mixed in a bone milling process with a plastic binder which is then poured on to a glass plate and spread to a predetermined thickness and allowed to dry, after which the material is placed on a ceramic plate and fired to bond the plastic and ceramic and burn out portions of the plastic whereupon electrodes are attached to the final thermistor.

10 Claims, N0 Drawings METHOD OF MAKING THIN FILM TI-IERMISTOR BACKGROUND OF THE INVENTION Thermistors are semiconductor resistance devices which exhibit resistance changes with temperature change have gained widespread use in various applications. They are used, for example, as temperature measuring devices and for temperature compensation in electronic circuits. Such thermistors are commonly made up of compositions including a mixture of various metal oxides. Conventionally the mixing of the compounds which go to make up the thermistor material is done mechanically. Such mixing does not always result in a uniform end product and thus thermistors with varying characteristics can result. In addition, conventional thermistors are generally bulky since they require special protection from the environment and thus are enclosed in glass beads or envelopes. Such construction does not readily lead to their rejection in size for inclusion in minaturized circuits.

Thus there is a need for a new method of manufacturing thin film thermistors such that intimate mixing of the materials which go to make up the thermistor occurs in order to achieve an end product which exhibits highly consistent characteristics. Further, such a thermistor should be able to be made as a minature component which is compatible for integration with integrated circuits.

SUMMARY OF THE INVENTION The present invention provides such a method of making a thin film thermistor which results in extreme uniformity in the mixing of the thermistor materials thereby resulting in an extremely consistent final product.

To achieve these results, the process is started using reagent grade materials of high purity. The materials, which are water soluble metal compounds, are mixed with water and allowed to intimately comingle in solution. This results in an aqueous chemical mixing which is the most efficient type of mixing. After a certain period of time of mixing in the water the solution is spray dried under pressure to obtain a fine powder whichrepresents the true composition desired for the thermistor material. The material is then oxidized at high temperatures to remove any sulphides or the like and to end up with only metal oxides. After oxidation the powder is mixed with a plastic binder in a bone milling process. The mixture is then placed on a glass plate over which an accurate doctor blade is run to leave a controlled thickness of thermistor material which is then allowed to dry. The dried material is then cut into squaresand attached to ceramic substrates by partially dissolving the binder, and the combination fired for a period of time and at a temperature which will cause portions of the plastic to burn out and the material to densify. The finished thermistor then has electordes placed upon it either by vacuum deposition or any other conventionalmeans. Such electrodes are placed on thethermistor in a manner which facilitates attaching external leads by pressure-compression, bonding, soldering etc.

The process is specifically designed to yield a monolithic fused construction which may be formed as a flat pack or a single chip. It thereby allows building a mi crominature or minature thermistor which can be integrated directly onto circuit boards as a discreet component and which is also compatible with present integrated circuit components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Inthe practice of the present invention, the first step is to determine the composition required of the final thermistor. Generally this combination will comprise a mixture of a plurality of metal oxides. Compounds containing the metals required are obtained in a reagent grade of high purity. These may comprise water soluble hydroxides, nitrates, sulphates or chlorides of the metals. These materials are then mixed in the desired percentages with distilled water and brought to a slow boil and held at such a temperature for several hours to assure complete reaction of all the involved compounds to result in a homogenous chemical solution. In practice the complete mixing is recognizable by the color of c the solution.

The solution is then spray dried under pressure at high temperature for example at 500F to result in a powder which is a true mixture of the desired compound and has a fine particle size in the range of 0.1 to 0.2 microns. This process forces out the excess as well as chemical water in a very rapid manner, thereby causing the resulting powder to retain the extremely homogenous mixture which was obtained by initially dissolving the compounds in water. A small capacity spray dryer may be used for this purpose into which the solution is injected at a high pressure and an elevated temperature in well known fashion. The temperature aids in accelerating the chemical process and the partial formation of oxides. Since some of the sulphides or nitrates, etc. may still remain in the compound, it is treated at a high temperature for an extended period of time to remove any of the nitrates and sulphates and to assure complete chemical combination and oxide formation.

The powder is then mixed with a plastic binder which may for example be Bakelite dissolved in methyl alcohol and amyl acetate. Various other organic binder combinations may equally well be used. It is only necessary that such binders lend themselves to being spread by a doctor blade in the manner described below and that they completely disintegrate without leaving a residue when fired at high temperatures. As noted below this is one of the steps in the process. This mixing is carried out in a bone milling process for approximately 48 to 72 hours to obtain an extremely fine consistency of the mixture. The proper amount of mixing is detected by measuring the pH value of the mixture .or the consistency thereof.

The mixture of plastic binder and semiconductor oxide powder is then spread on a glass plate and a doc tor bladedrawn over the viscous liquid to cause a predetermined accurate thickness to remain in a layer on the glass plate. The mixture is allowed to dry thereon, after which the resulting film is cut into appropriate sizes for example 1 inch squares. These squares are then placed on a ceramic substrate using asolvent such as methyl alcohol and amyl acetate being used to partially dissolve the binder and cause the film to be bonded to the substrate. The ceramic substrate and film are then fired at high temperature to cause intimate bonding of the film with the substrate and burning out of the plastic resulting in a densification of the thermistor oxide material.

The final step of the process is the attachment of V electrodes. This may be done in a vacuum deposition chamber using a mask to cover a portion of the thermistor upon which electrodes are not to be deposited. Similarly other conventional means of attaching electrodes may also be used. The resistance of the finished thermistor may be controlled by controlling the amount of thermistor material remaining exposed after electrode deposit. Since the resistance of the thermistor will be a function of its resistivity times its length divided by its area. The area is the thickness times the width both of which are controlled; the thickness by the doctor blading and the width by the dimension to which the material is cut. Thus by adjusting the length of material remaining between electrodes deposited on each side, the resistance of the thermistor may easily be controlled. In addition, depositing electrodes in this manner permits easily attaching external electrodes through which the thermistor may be interconnected with other circuits. Preferably silver or gold electrodes will be deposited on the thermistor and thereafter external electrodes attached by pressure-compression, bonding or soldering in conventional fashion.

EXAMPLE To manufacture a thermistor according to the present invention the following steps were followed. Water soluble materials of a high purity reagent grade were mixed in the following percentages by weight with the indicated amount of water.

The mixture was then heated at a low boiling point for approximately 4 6 hours until the mixture has a color of dark 'maroon. The solution was then spray dried under pressure at a temperature of approximately 260C. The resulting powder was processed at a high temperature of 900C for approximately 24 48 hours. After the high temperature oxidation the powder was mixed for approximately 48 hours in a bone milling process with a plastic binder comprised of XYSG Bakelite dissolved in methyl alcohol and amyl acetate. Approximately 36.5 grams of XYSG Bakelite and 130 ccs of methyl alcohol along with 130 ccs of amyl acetate were used. The mixture was mixed until it had a consistency of molasses or thick cream which is a viscosity range of 400-1200 centipoise-seconds whereupon it was poured onto a glass plate and a doctor blade drawn over it to leave a thickness of 0.0075 inches. The mixture was allowed to dry to form a thick film of 0.002 inches thickness and then cut into 1 inch squares. The squares were then attached to ceramic substrate plates with a mixture of methyl alcohol and amyl acetate applied to aid in bonding of the film to the plate. The combination was tired for approximately one hour at a temperature of l250C to result in good bonding between the substrate and the thermistor film and causing densification of the thermistor. Adjustment in firing time and temperature can be used as a means of maintaining tighter control on the resistivity of the finished thermistor. The finished product was then placed in a vacuum deposition chamber and had gold electrodes deposited on two sides to a thickness of less than 1 mil (0.0011) and leaving a length of 10 mils of thermistor material between the electrodes. For this material with a resistivity of 3Mohm/ at room temperature this length and a width of mils resulted in a resistance of300 Kohm.

Thus an improved method of manufacturing a thin film thermistor has been disclosed. Although a specific embodiment has been described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention which is intended to be limited solely by the appended claims.

What is claimed is:

l. A method of manufacturing a thin film thermistor comprising:

a. dissolving a plurality of compounds containing the metals desired in the finished thermistor in water;

b. boiling said mixture until intimate chemical combination of the materials results; c. spray drying the solution to obtain a fine powder;

d. heating the powder obtained at a high temperature until complete oxidation of the metals results;

e. mixing the powder with an organic binder in a bone milling process;

f. spreading the resulting mixture on a smooth surface at a predetermined thickness;

g. after drying, cutting the film into predetermined h. placing said out film on ceramic substrates;

i. firing the combination of the film and substrate to result in bonding and densification; and

j. attaching electrodes to the finished thermistor.

2. The invention according to claim 1 wherein said organic binder material comprises Bakelite dissolved in methyl alcohol and amyl acetate.

3. The invention according to claim I wherein the compounds dissolved in water consist of magnesium sulphate, cobalt sulphate, copper sulphate and nickel sulphate.

4. The invention according to claim 2 wherein said compounds consist of 27.2 percent by weight of magnesium sulphate, 61.9 percent by weight of cobalt sulphate, 9.9 percent by weight of copper sulphate and 1 percent by weight of nickel sulphate.

5. The invention according to claim 1 wherein said solution is spray dried under pressure at the temperature in the range of 500-700 F.

6. The invention according to claim 5 wherein said resulting powder is fired at a temperature in the range of 900C for approximately 24 to 48 hours.

7. The invention according to claim 1 wherein said thin film is deposited to be approximately 0.0075 inches thick.

8. The invention according to claim 1 wherein said thin film and substrate is fired for approximately one hour at a temperature in the range of 1250 C.

9. The invention according to claim 2 wherein a portion of the plastic binder is dissolved when placing said cut film on said substrate.

10. The invention according to claim 1 wherein said electrodes are attached by vacuum deposition. 

2. The invention according to claim 1 wherein said organic binder material comprises Bakelite dissolved in methyl alcohol and amyl acetate.
 3. The invention according to claim 1 wherein the compounds dissolved in water consist of magnesium sulphate, cobalt sulphate, copper sulphate and nickel sulphate.
 4. The invention according to claim 2 wherein said compounds consist of 27.2 percent by weight of magnesium sulphate, 61.9 percent by weight of cobalt sulphate, 9.9 percent by weight of copper sulphate and 1 percent by weight of nickel sulphate.
 5. The invention according to claim 1 wherein said solution is spray dried under pressure at the temperature in the range of 500*-700* F.
 6. The invention according to claim 5 wherein said resulting powder is fired at a temperature in the range of 900*C for approximately 24 to 48 hours.
 7. The invention according to claim 1 wherein said thin film is deposited to be approximately 0.0075 inches thick.
 8. The invention according to claim 1 wherein said thin film and substrate is fired for approximately one hour at a temperature in the range of 1250* C.
 9. The invention according to claim 2 wherein a portion of the plastic binder is dissolved when placing said cut film on said substrate.
 10. The invention according to claim 1 wherein said electrodes are attached by vacuum deposition. 